Rechargeable lithium-ion batteries (LIBs) have been attracting much attention in the past few decades. LIBs with high power and energy density are highly desirable in order to meet the increasing demand for energy storage, in particular, electric vehicles. Currently, LIBs exclusively use carbon as negative-electrode materials for its good cycling performance based on the intercalation/de-intercalation mechanism of Li storage. However, the theoretical capacity of graphite at 372 mA h g−1 (based on LiC6) has almost been achieved.
On the other hand, the intercalation of lithium into graphite mainly occurs at low potential close to zero V (vs. Li/Li+). Accidental overcharge at high currents may lead to the possible formation of lithium dendrites that short circuit anode and cathode and cause thermal runaway or even a fire. Recently, much attention has been devoted to the development of carbon-alternative negative-electrode materials, which must have higher specific capacity and better safety performance than the widely adopted carbon anode.
Various metal oxides have been extensively explored as carbon-alternatives, in particular, magnetite Fe3O4. Fe3O4 has a theoretical capacity of 927 mA h g−1 and its potential of lithium insertion based on the conversion-type mechanism is significantly higher compared to that of carbon. Other advantages are low cost, abundance, environmental friendliness, and especially the high electrical conductivity at room temperature of about 2.5×102 S cm−1 among all metal oxides.
High electrical conductivity is rarely observed in other metal oxides investigated for application in LIBs (e.g. α-Fe2O3 has an electrical conductivity of rv10−4 S cm−1, which is six orders of magnitude or ×10−6 lower than magnetite). High conductivity is highly desirable for electrodes in LIBs to facilitate charge transfer. However, as one of the conversion-type negative-electrode materials, the volume expansion (˜200%) of magnetite is much larger than that of insertion-type negative-electrode materials (such graphite) upon lithium insertion. This huge volume variation or pulverization could cause disintegration of the electrode and lead to poor cycling performance.
This poor cyclability becomes one of the obstacles to commercialize Fe3O4 as negative-electrode materials in LIBs. On the other hand, based on the conversion-type lithium storage mechanism, metallic iron (Fe0) nanograins will be generated through electrochemical reduction. Fe0 nanograins are highly reactive toward the organic electrolyte. The irreversible reactions on the surface of Fe0 nanograins with the electrolyte could also cause poor cycling performance. To address the poor cycling performance of Fe3O4, one strategy is to adopt nanoscale materials to buffer the volume variation during the charge-discharge process. The other strategy is to add or coat with carbon to minimize the exposure between Fe0 nanograins and organic electrolyte as well as to increase the electrical conductivity. For example, Fe3O4-carbon composites have been demonstrated to achieve a certain level of success in terms of electrochemical performances. The composites include Fe3O4-carbon nanospindles, Fe3O4-carbon nanorings, and C-encapsulated Fe3O4 nanoparticles homogeneously embedded in porous graphitic carbon nanosheets. Also reported have been a series of nanostructured iron oxide based anode materials for LIBs, such as carbon coated Fe2O3 nanorods, nanocubes, microboxes, nanotubes, nanodiscs, nanospheres, hollow microspheres of Fe3O4, nanohorns on CNTs, and Fe3O4 nanospheres with carbon matrix. Therefore, Fe3O4 could find promising application as negative electrodes in LIBs.
Moreover, manufacturing processes for electrodes for LIBs currently take multiple steps including mixing, roll-coating, compressing, and drying and involve the use of organic chemicals, some of which can be toxic, as binders and solvents.
It has been a challenge to develop nanoparticles that allow for exploitation of the advantageous properties of iron compounds for use in lithium ion batteries, as well as methods for decreasing the complexity and time of manufacture for electrodes.